A professor at twenty-one and member of the Napoleon's Egyptian expedition at twenty-six, Geoffroy Saint-Hilaire was a man of one idea, which he formulated when he was twenty-four. Nature, he thought, had formed all living beings with one single plan. This was a revolutionary idea--and one vigorously opposed by Geoffroy's colleague Georges Cuvier, a great anatomist and one of the giants of French science. In 1830, their long-running disagreement...